Dynamics of Oceanic Jets in the Presence of Zonally Varying Bottom Topography

Oceanic alternating jets are seen to possess transient features in the presence of bottom topography and the dynamics tend to be very complex involving a variety of interactions. In this work, the behaviour of jets has been studied in the presence of a simple topography, which increases linearly in the zonal direction. A two-layer quasi-geostrophic model is used for the study, which is forced with a uniform background flow in the upper layer. Unlike the flat-bottom case, jets are tilted from the zonal direction and drift meridionally with a constant speed. In addition, the system becomes unstable and purely zonal large-scale modes appear due to the zonal asymmetry. This happens in cases of weak friction only and the zonal modes also propagate meridionally, but opposite to the jets. Looking at the kinetic energy spectrum, it seems that these zonal modes receive energy from unstable jets through nonlinear interactions. Linear instability analysis is performed to study these instabilities with the focus on the effects of eddy viscosity and bottom friction. We find that strong friction suppresses small-scale instabilities. Hence, it is possible that alternating jets interact nonlinearly with small scales and feed energy to large-scale zonal modes, which are otherwise suppressed due to strong friction.